How to install lsystem add-on?
As stated in the readme.md, you would have to copy the lsystem folder to your add-on directory:

Copy lsystem directory over to F:\SteamLibrary\steamapps\common\Blender\2.80\scripts\addons or wherever your blender installation is.


Linux: $HOME/.config/blender/&lt;version&gt;/
OSX: /Library/Application Support/Blender/&lt;version&gt;/
Windows: %USERPROFILE%\AppData\Roaming\Blender Foundation\Blender\&lt;version&gt;\


Link to the manual for reference: https://docs.blender.org/manual/en/latest/advanced/blender_directory_layout.html#blender-s-directory-layout